The virtual service class of XCube namespace is an actual definition for business logic. The sub class of XCube_Service is able to connect with other modules, other sites and smart clients like a mobile phone. Developers will use XCube_Service to relation between modules. By that, site owners can publish API. In other words, XCube_Service is an abstract layer for module relationship and site relationship.
Shade is example as XCube namespace. It doesn't have the compatibility with Legacy. If a developer want to port his module for Shade, he has to make his library well. But, cubson isn't good for it. That's problem. Cubson should generate common modules of XCube layer as far as possible. If cubson will come to do it, cubson users can port some parts of his module as library for Shade. And, even if he can't do it, he may be able to run the web service on Legacy. At least, cubson has to come to generate the implementation of XCube_Service at Legacy.